The core rule
Matcha degrades fastest when it meets air, heat, light, and kitchen humidity. Good storage is mostly about reducing those exposures consistently rather than chasing perfect rituals. If you open a large bag slowly and leave it near the stove, the tea will flatten no matter how premium it was at purchase.
Best everyday practice
Keep unopened matcha cold if you will not use it soon. Once opened, seal it tightly, keep it away from heat and light, and use it steadily instead of opening multiple containers at once. Smaller tins are easier to keep fresh than large bags unless you are a heavy user.
When big bags make sense
Large bags make sense for frequent latte drinkers or households that move through matcha quickly. They are less ideal for casual straight-drinking buyers, because by the time the last portion is opened the tea may have already lost some of the sweetness and aroma you paid for.
What freshness loss tastes like
The first signs are usually flattened aroma, duller color, and bitterness arriving faster than it should. If a tea once tasted creamy and now feels dry or generic, storage is often the reason.